This invention relates to the design of an information transport system for providing data services over existing telephone lines without interference with existing voice services supported by the telephone network. In particular, the system achitecture permits the invention to support multiple types of subscriber data services.
Telemetry and information transport is useful in providing passive services, active services and interactive services. The provision of such services is of particular interest where a multiplicity of subscribers are engaged with one or more providers of such services. These services will now be described in further detail.
In passive services, the data to be transmitted over a network is of relatively "bursty", i.e. small quanta of data with relatively low frequency of occurrence, particularly in those situations wherein the critical function of time for retrieval is low. For example, the data generated by utility meters, such as water, gas, and electric meters is generated locally, at the site of a subscriber to the telemetry service, changes slowly and is stored on site and need not be retrieved at intervals, generally less frequent than once an hour. Data of passive services is further characterized in that the rate of retrieval of the data is determined by the user of the data, such as the billing company, rather than the originator of the data, namely, the meter with its encoder.
Attempts to retrieve such information have been made at various times by various equipment manufacturers. Systems which have been developed for retrieval of such data can be classified as (1) dial-up systems, (2) call-discrimination systems, and (3) polled systems. These systems have generally been implemented by use of existing telephone switches to connect the user of the data with the originators of the data. Alternatively, dedicated full time lines are used, for example, with industrial customers of many utilities.
In the use of a switched telephone network for polled and call-discrimination systems, there is the possibility of overloading the traffic handling capability of the telephone exchange, as well as operating with delays inherent with the establishment of a connection through the network. Currently available polled systems are designed for either specific applications, such as a single user, or are designed with interfaces which are capable of interfacing only with a single type of encoder. The expandability of such systems and the capability to interface with more than one type of encoder is a significant limitation in the use of the concurrently available systems. In the case of dedicated lines to gather the data there is an economic disadvantage of full time line charges.
At the present time, all polled systems require certain common equipment located at the telephone exchange. This equipment must be connected to the main distribution frame manually and, in many cases, such connection requires a complete rewiring of the main distribution frame. The cost of such rewiring frequently requires two flexible connections where one previously existed. The added cabling and wiring in the limited space of the distribution frame presents a major constraint in many applications. Each one of the common equipment systems is capable of being enlarged only to the capacity of most medium sized telephone exchanges (10,000 lines) and do not lend themselves to facile expansion or networking of the system.
Active services relate to the situation wherein the information generated at the subscriber premises determines the need and the timing with which the data must be communicated to a remote site, as in the case the reporting of an alarm. In contradistinction to the passive services, the active services frequently require two-way communication from the subscriber premises to the provider of service, as would be the case with control functions such as the management of energy peak loads at the site of the subscriber.
Currently, there are two basic systems for handling the task of alarm reporting, namely, (1) dial-up systems (2) polled systems over dedicated lines.
Alarm reporting with dial-up systems is probably the most widely available technique. The major drawback is the ease with which the alarm reporting function can fail, without notification of the failure, if breakdowns on the transmission line or the equipment itself occur.
Polled systems installed over dedicated telephone lines are also in current use. The polled systems require dedicated subscriber line pairs for operation, and represent an extra economic burden to the telephone plant. Also, their cost, due to the inherent nature of the service, is high.
Telemetry or information transport systems for control functions, when implemented in the framework of a network, have been done to the present time, generally, over power line, carrier based systems.
Interactive services are those wherein human intervention occurs at the subscriber premises. Thus, a human being is the originator, service requestor and an active participant in the transaction as would occur, by way of example, in the transmission of video text.
A system for providing data communication between a human operated terminal and a remote service provider is implemented usually by a switched telephone network wherein dial-up type modems are employed to transport the information between the human operated terminal and the remote service provider. Such a system requires the establishment of a connection via the switched telephone network which remains in place throughout the duration of the time interval during which the interactive service is provided. During such interval, only minor effective utilization of the communication channel of the telephone network is attained. Also, during this period of time, the network and the subscriber pair in use remain unavailable to the subscriber for normal voice telephone service.
Thus, a problem exists in that no system is presently available to provide an overlaid data network with existing subscriber line pairs, and which interconnects the individual subscriber to the data services. Such is the case also with systems providing the foregoing active service and passive service.